A Cambodian court has convicted opposition leader Sam Rainsy in absentia on two counts of criminal defamation, a move that could keep the government critic in self-imposed exile for years. Human rights groups accuse the government of using the judiciary to silence opponents.

Sam Rainsy was sentenced to 18 months in prison and fined approximately $14,000 after being found guilty by the Phnom Penh Municipal Court of defaming Prime Minister Hun Sen and National Assembly President Prince Norodom Ranariddh.

The verdict was expected. Critics of the prime minister accuse him of using the courts to silence his opponents.

Opposition member Mu Sochua called on the international community to help broker a political truce that would allow Sam Rainsy to return home. She said the court ruling stole his chances of participating fully in national elections in 2008.

"It would prevent Rainsy, the leader of the opposition, to have a complete role, presence for the campaign towards the election. This would really impair the election," she said.

Prince Ranariddh filed a criminal defamation suit in 2004, after Sam Rainsy accused him of accepting bribes to form a coalition government with the prime minister's ruling Cambodian People's Party.

The second defamation case was a countersuit that Hun Sen filed last September. Sam Rainsy had filed a suit, which he ultimately lost, accusing the prime minister of masterminding a grenade attack on an opposition rally in 1997.

Judge Chiv Keng said Sam Rainsy had acted with ill will and used his influence to tarnish the reputation of the country's leaders. He handed down a pair of twin sentences, ordering Sam Rainsy to serve nine months in prison in each of the cases, pay approximately $2,000 each in fines, and $5,000 each in compensation to the plaintiffs.

Sam Rainsy went into exile last February after the parliament stripped his parliamentary immunity. Lawyer Som Chandina, who represented him at trial, argued that the defendant had only expressed ideas for social change.

The Cambodian government insists Hun Sen and Ranariddh filed the lawsuits to defend their reputations, but rights groups consider the litigation a form of political warfare on dissenters.

New York-based Human Rights Watch this week called on the government to end "spurious, politically motivated criminal lawsuits as a way to silence its critics."

At least seven critics of a controversial new border treaty with Vietnam also face criminal lawsuits ordered by Hun Sen. In October, a radio journalist and union president who publicly opposed the treaty were arrested, and are being held without bail on charges of defamation and incitement. Human Rights Watch, the United States and the European Union have all called for their immediate release.

Another opposition lawmaker was jailed in February, and yet another fled the country, like Sam Rainsy, for fear of arrest.

Members of the Sam Rainsy Party are struggling to maintain a united front as they face local and national elections over the next three years.